Clarify reuse of parity binary

README.md

@@ -21,6 +21,7 @@ For the optional bloxberg node build (`INSTALL_EVM=bloxberg`), additionally thes
 - rustup (1.24.3)
 - clang (12.0.1)
 - cmake (3.21.2)
+- openethereum (1.47.0)
 
 
 ## Installation settings
@@ -48,7 +49,14 @@ If you want to build the bloxberg mode executable (be warned, that's a long wait
 
 ```
 export INSTALL_EVM=bloxberg
-``` 
+```
+
+If you wish, you can use an existing openethereum executable instead by setting:
+
+
+```
+export OPENETHEREUM_PATH=<path_to_binary>
+```
 
 
 ## Installation